1. Field of the Invention
This invention relates to ultrasonic diagnostic imaging, and more particularly, to an ultrasonic imaging system that produces a spatially compounded image by an image registration method that correlates movement between consecutive image frames.
2. Description of Related Art
Ultrasonic imaging techniques are commonly used to produce two-dimensional diagnostic images of internal features of an object, such as a human anatomy. A diagnostic ultrasonic imaging system for medical use forms images of internal tissues of a human body by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that travel into the body. The ultrasonic pulses produce echoes as they reflect off of body tissues that appear as discontinuities or impedance changes to the propagating ultrasonic pulses. These echoes return to the transducer, and are converted back into electrical signals that are amplified and decoded to produce a cross-sectional image of the tissues. These ultrasonic imaging systems are of significant importance to the medical field by providing physicians with real-time, high resolution images of the internal features of a human anatomy without resort to more invasive exploratory techniques, such as surgery.
The acoustic transducer which radiates the ultrasonic pulses typically comprises a piezoelectric element or matrix of piezoelectric elements. As known in the art, a piezoelectric element deforms upon application of an electrical signal to produce the ultrasonic pulses. In a similar manner, the received echoes cause the piezoelectric element to deform and generate the corresponding electrical signal. The acoustic transducer may be packaged within a handheld device that allows the physician substantial freedom to manipulate the transducer easily over a desired area of interest. The transducer would then be electrically connected via a cable to a central control device that generates and processes the electrical signals. In turn, the control device transmits the image information to a real-time viewing device, such as a video display terminal (VDT). The image information may also be stored to enable other physicians to view the diagnostic images at a later date.
A drawback of conventional ultrasonic imaging systems is that the image frames are susceptible to various undesirable phenomena, such as ultrasound image "shadowing" or "speckle." Ultrasound shadowing occurs when the ultrasonic pulses produce shadows by reflection from tissue regions of disparate density that effectively prevent the ultrasonic pulses from reaching a region of interest and producing an echo return. When this happens, information regarding the region of interest is not collected, and the imaging system cannot produce a complete image of the region. To the viewer of the image, the image shadowing appears as dark patches that cover portions of the region of interest. The dark patches tend to mask or otherwise obscure small image detail and contrast differences within the image, making diagnostic analysis of the image more difficult and thus less reliable.
The image shadowing phenomenon and its solution are analogous to a similar problem with the use of a flashlight to illuminate an object of interest. If the object of interest lies either partially or completely within the shadow of another object, the object of interest cannot be completely seen; however, the flashlight can be repositioned to illuminate the object of interest from another direction in order to avoid the shadow of the blocking object. This technique represents an image averaging technique. If all the information received by illuminating the object of interest from different viewing angles were combined into a single image, then a complete, multidirectional image of the object of interest could be obtained. Within the ultrasound domain, the acoustic transducer can be repositioned so that the region of interest is imaged from several different directions. By combining these different ultrasonic images together, the image shadowing would be filled in and a more complete image of the region of interest could be obtained. This process of combining multiple images into a single image is commonly referred to as spatial compounding.
Ultrasound image speckle is an artifact caused by coherent image formation. A speckle pattern is generated by interference among echoes from a large number of randomly distributed ultrasound scatterers within the imaged area. The ultrasound speckle appears as random noise in the image, and its existence makes ultrasound diagnosis more difficult, especially in areas having low contrast and small-sized lesions. It has been observed that by varying the position of the ultrasound imaging transducer slightly, the image of the lesions and other deterministic targets in the imaged region is relatively unchanged, but the speckle pattern will change substantially. A spatially compounded image obtained by averaging several images taken from slightly different spatial locations would have significantly reduced ultrasound speckle.
There are two known methods for generating a spatially compounded ultrasound image. A first technique utilizes a conventional transducer that is moved to various locations to acquire several different images. The transducer location is accurately measured by sensing devices in order to register each of the discrete image frames together into a single compound image. An example of a compound image-scanner utilizing angular sensing devices on an arm assembly is disclosed in U.S. Pat. No. 4,431,007, to Amazeen et al., for REFERENCED REAL-TIME ULTRASONIC IMAGE DISPLAY. In practice, however, the arm assembly is awkward and inflexible to operate, and the sensing devices add significant complexity and cost to the ultrasonic imaging system.
A second technique uses a large aperture phased array transducer to generate two or more images at slightly differing viewing angles from a fixed transducer location. The images can be readily combined since their relative orientation is generally known, or can be ascertained. This technique produces acceptable results when the region of interest is relatively shallow below the skin surface, as very little transducer movement is necessary. The technique is less effective for deeper level imaging, however, since the phased array transducer must necessarily be very large in order to produce viewing angles between individual image frames that are large enough to yield acceptable results.
Thus, a critical need exists for a method and apparatus for spatially compounding a plurality of discrete image frames produced by an ultrasonic imaging system into a single image in a manner that overcomes the drawbacks of the prior are techniques. Of particular importance, the method and apparatus should be compatible with modern handheld ultrasonic transducers without encumbering the handheld transducers with position sensing devices that undesirably increase the cost, weight and complexity of such imaging systems.